Valve actuator system

ABSTRACT

A pneumatic-hydraulic proportional valve actuator system is provided having a relatively simple and adjustable means of achieving two stroking speeds by the hydraulic actuator, high speed in normal operation and low speed for small position adjustments and for the final part of any large position adjustments. This prevents overshoot and instability of the actuator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to valve actuator systems andparticularly to valve actuator systems using two stroking speeds for theactuation, fast and slow, depending on the degree of required positionadjustment of the actuator.

2. Description of the Prior Art

Known valve actuators utilize a single stroking speed and are usuallypneumatic, electrohydraulic or electromechanical. Each has its owndeficiencies which the present invention overcomes. Pneumatic actuatorsare necessarily large in size to provide the required force or thrustand suffer from dynamic instability because of the compressible natureof the pneumatic fluid, as well as from overshoot and instability due tothe single stroking speed attempting to effect a small actuator positionmovement. Electromechanical and electrohydraulic actuators althoughhaving less sources of instability are usually slow in operation incomparison to the pneumatic actuators.

Examples of such known prior art devices may be found in the followingU.S. Pat. Nos. 3,603,083 and 2,938,347.

SUMMARY OF THE INVENTION

The present invention solves the mentioned problems associated withprior art devices as well as others by providing a pneumatic-hydraulicvalve actuator system which is compact in size, fast in operation,capable of developing a high thrust, and fire-safe in hazardousenvironments.

The system utilizes a hydraulic piston to actuate the valve. The pistonis connected through a three-position four-way valve to a hydraulicaccumulator which is supplied by an air-actuated hydraulic pump. Anorifice with a free flow by-pass is mounted in line between the pistonand the accumulator to selectively allow two-speed, fast or slow,actuation of the piston. The system controllably supplies theaccumulator hydraulic pressure to the piston either through the by-passto provide high volume fluid flow and speedy valve actuation for largevalve movements or through the constricting orifice to providerestricted volume fluid flow and slow valve actuation for small valvemovements. This prevents overshoot of the desired valve positionresulting in a fast and stable valve actuation system.

In a specific embodiment of the invention the valve is controlled by apneumatic positioner which compares an input control signal of desiredvalve position with a feedback signal from the piston of actual valveposition to appropriately control the valve to balance the input andfeedback signals. If the initial error signal is large, the system willopen a valve in the by-pass to allow fast valve actuation until theerror signal is within a predetermined range at which time the systemwill close the valve in the by-pass preventing flow therethrough andforcing the flow through the orifice to allow the valve to slowly cometo the desired position where the signals are balanced. The valve in thesignal balanced position disconnects the accumulator from the piston tohydraulically lock the piston and valve in the predetermined position.Thus a loss in supply air would lock the valve in the last predeterminedposition since the air-actuated pump and positioner do not affect thenull position of the piston.

In view of the foregoing it will be seen that one aspect of the presentinvention is to provide a two-speed valve actuator system which isstable.

Another aspect of the present invention is to provide a two-speed valveactuator system which will lock the valve in place at desired valvepositions.

These and other aspects of the invention will be more clearly understoodupon review of the following description of the preferred embodimentwhen considered with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the valve actuator system of thepresent invention.

.Iadd.FIG. 2 is a schematic representation of an alternate actuatorsystem of the present invention. .Iaddend.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, FIG. 1 shows a pneumatic-hydraulic valveactuator system 10 having a hydraulic piston 12 which is used tomodulate a valve 14 by way of a physical coupling 16 connecting a stem18 of the hydraulic piston 12 to a valve stem 20 of the valve 14. Thehydraulic piston 12 is selectively pressurized by an accumulator 22which is connected to this piston 12 by way of a three-position four-wayvalve 24. The valve 24 is shown to be in the null position with supplyline 26 from the accumulator 22 being closed to supply line 28 whichfeeds one side 30 of the piston 12. Similarly, the other side 32 of thepiston 12 is blocked by this position of the valve 24 from exhaustingany fluid from the side 32 along exhaust line 34 to exhaust line 36which feeds the hydraulic reservoir 38 through a filter 40. With thevalve in this position the piston 12 is incapable of any movement sincethe piston 12 is hydraulically locked thereby assuring a stableposition.

The accumulator 22 is charged up to approximately 3,000 psi pressure byan air-actuated hydraulic pump 42 such as the Haskel Model P-35CAir-Actuated Hydraulic Pump. The pump 42 draws hydraulic fluid from thereservoir 38 along line 44 and exhausts it along line 46 to theaccumulator 22. The pump 42 is operated by regulated air supply ofapproximately 50 psi which is supplied to the pump 42 from air supplyline 48 regulated down to 80 psi by the regulator 50 and supplied to thepump 42 along line 52. The pump 42 maintains its output to theaccumulator 22 until it has charged up the accumulator 22 tosubstantially 3,000 psi at which time the pump stalls itself out sinceit is a non-continuously operating pump which may be started and stoppedin any number of known ways.

The position of the valve 14 is monitored by a penumatic positioner 54such as the Bailey Controls Company Model P58-4 Pneumatic Positioner.The positioner 54 compares the input signal supplied along line 56indicative of desired valve 14 position with a feedback signal of actualvalve position supplied to the positioner 54 along feedback line 58. Inthis particular instance the feedback line 58 is an actual mechanicallinkage between the piston stem 18 and the positioner 54. As long asthere is no difference between the input control signal originating fromline 56 and the feedback signal from line 58, the positioner 54establishes a φ psi pneumatic output signal to lines 60 and 62 whichconnect opposite ends of the valve 24 thereby maintaining the valve 24in the balanced or null position shown in the drawing. Should the inputsignal 56 change to indicate a different valve 14 position, thepositioner 54 will detect a difference between the input signal 56 andthe feedback signal 58 causing either line 60 or line 62 to bepressurized depending upon the polarity of the signal difference betweenthe input signal and the feedback signal. Assuming that the differencebetween the input signal 56 and the feedback signal 58 indicates a valveclosing, the line 62 will remain at φ psi while line 60 is pressurized.This will cause the valve 24 to switch to the position connecting supplyline 26 to supply line 28 and exhaust line 34 to exhaust line 36. Thepressure of the accumulator 22 is now supplied by this connection to theside 30 of the piston 12 through a restrictor and by-pass assembly 64,whose control or function will be discussed later, while the other side32 of the piston 12 is now free to be exhausted into the reservoir 38.This unbalances the piston 12 and causes it to move down towards thevalve at one of two speeds as determined by the assembly 64. As thepiston 12 moves, the feedback signal 58 changes until it balances thesupply signal 56 at which time the line 60 will vent through thepositioner 54 causing the valve 24 to return to the null positionhydraulically locking the piston 12 in the new position. It will be seenthat the present system provides proportional valve actuation whereinthe movement of the stem 18 is directly proportional to the input signal56 indicative of desired valve position.

Since the positioner 54, as all instruments, has a certain dead band andsince the speed of the piston movement is extremely fast due to the highpressures provided by the accumulator 22, the restrictor and by-passassembly 64 is placed in the line 26 to control the flow of hydraulicfluid into either of the piston sides 30 or 32 or restrict the flow fromsides 30 or 32 to thereby control the speed of piston movement 12 in aspeedy and stable manner. Note that the by-pass assembly can be in line26 .Iadd.as seen in FIG. 1 .Iaddend.or 36 .Iadd.as seen in FIG.2.Iaddend., when in line 36 the two-way valve 66 responds faster,because of less pressure to work against. The assembly 64 is controlledby a two-position normally closed valve 66 to provide a first fast speedfor large piston 12 movement and a second slow speed for small pistonmovement. This two-speed movement makes the system both fast and stableand allows the piston 12 to come to the new position required by theinput signal 56 in the minimum amount of time without causingoscillations or overshooting of the piston 12 around the desiredposition.

The restrictor and by-pass assembly 64 includes a restrictor orifice 68mounted directly in line 26 or 36 to control the volume of fluid flowfrom the accumulator 22 to the hydraulic cylinder and piston 12. Thisflow will be relatively slow due to the predetermined restrictor andwill thus result in a relatively slow piston 12 reaction and movement. Aby-pass line 70 is connected across the restrictor orifice 68 with thenormally closed two-position valve 66 mounted in the by-pass line 70 toprevent fluid flow across the restrictor orifice 68 in the normal valve66 position. In the normal position of valve 66, fluid has to flowthrough the restrictor. The valve 66 is controlled between its open andclosed position by a second two-position normally closed valve 72 whichis a high pressure relay normally closed and which is relatively smalland quick-acting in comparison to the valve 66 which is relatively largein order to provide a large fluid flow capacity through the by-pass line70. The control valve 72 is moved between its normally opened and closedposition by a high pressure selector switch 74 connected across lines 60and 62.

Turning to the operation of the forementioned devices, it will be seenthat when the valve 14 is in a balanced position the pressure signalsapplied to lines 60 and 62 are φ psi. Thus causing the selector switch74 to be at φ psi. Thus no pressure or force is applied to the highpressure relay 72 along line 80 from the switch 74 maintaining the highpressure relay 72 in the closed position. The closed position of therelay 72 prevents supply air flowing from line 82 to reach the valve 66.Whenever a large movement of the valve is required, a great error signalwill result between the input signal and the feedback signal. As wasmentioned, this will cause one of the lines 60, 62 to become pressurizedwhile the other line will remain at φ psi. Note that the selector switch74 always lets the higher pressure in line 60 or 62 flow through, andacts like a check valve, never letting pressure feed back into line 60or 62. This pressure in line 60 or 62 will be sensed by the switch 74and the net pressure will be transmitted along line 80 to the controlvalve 72. If this pressure is sufficiently large, the control valve 72will be switched to its open position allowing supply air to flow alongline 82 to the valve 66. Valve 66 will now be switched to the openposition and will allow fluid flow from the accumulator 22 through theby-pass line 70 to flow to the valve 24 and therefrom to the hydrauliccylinder and piston 12 in an unhindered manner. This will provide speedymovement of the piston 12. The movement of the piston 12 will be sensedby the feedback means and transmitted to the positioner 54 along line58. As the piston 12 approaches the desired position, the positioner 54will drop the pressure applied to either line 60 or 62 producing aconstantly lower net pressure to the switch 74 and therefrom along line80 to the control valve 72. At a predetermined point of net pressurewhich is coordinated and preset to a desired percentage of full scaleerror signal travel, the control valve 72 will no longer be maintainedin the open position by the net pressure along line 80. When thishappens the control valve 72 will revert to its normally closed positionpreventing further supply air flow along line 82. The lack of supply airin line 82 will now prevent valve 66 from remaining in its open positionand the valve 66 will close preventing flow of fluid from theaccumulator 22 along the by-pass line 70. Flow of fluid from theaccumulator 22 will now have to occur through the restrictor orifice 68.This will significantly cut down the volume of fluid flow and will causea decrease in the speed of piston 12 travel. This is a desired conditionsince the piston 12 is now close to its desired position. Thus overshootof the desired position will be prevented and the remaining movement ofthe piston 12 will occur in a manner approaching a critically dampedcondition resulting in stable and speedy control. When the desiredpiston 12 and thus valve position is reached the error signal becomeszero and the positioner 54 again balances the pressure in lines 60 and62 resulting in a zero net pressure applied to the relay 74. Reset ofthe valve 66 is accomplished by an adjustable bleed orifice 84 connectedto line 82 while reset of the control valve 72 is accomplished throughadjustable bleed valve 86 connected to line 80. Should the next errorsignal be of a relatively small magnitude indicating a small piston 12position change, the unbalance in pressure between lines 60 and 62 wouldbe small and the net pressure and force along line 80 to control valve72 would be insufficient to change it from its closed to open position.Thus supply air would be prevented from flowing through line 82 andvalve 66 would remain in the closed position preventing flow alongby-pass line 70. Thus for small error signals the movement would be onlyslow since flow would occur from the accumulator 22 always through therestrictor orifice 68. The two-speed operation of the piston 12 willoccur only in situations where large error signals are developed.

Should the signal 56 be changed to now indicate a valve 14 up positionthe line 60 would remain at φ psi, while line 62 would be pressurized bythe positioner 54. This would drive the valve 24 up into its thirdposition causing the line 26 to be connected to the line 34 while theline 28 is connected to the line 36. The effect of this would be topressurize the opposite side 32 of the piston 12 while the first side 30of the piston 12 is vented to the reservoir 38. The unbalance of thepiston 12 would now be in the opposite direction of that previouslydisclosed and the piston 12 would now move up in the manner discussedpreviously causing the valve 14 to also be moved in the up position inthe speedy and stable manner effected by the two-speed valve actuator ofthe present invention.

In the event that air supply is lost to the system 10 the piston 12still remains hydraulically locked. The piston 12 may be manually movedby depressing either side of the valve 24 to connect the piston 12 tothe accumulator 22. The accumulator 22 maintains its charge of 3,000 psieven though the pump 42 is no longer running since the pump 42 is not acontinuously operating pump but one that charges up the accumulator andthen stalls itself out. Even without any air being supplied to thepositioner 54 the valve 24 maintains its middle null position since bothlines 60 and 62 are equally affected by the loss of supply air to thepositioner 54 to maintain the valve 24 in balance.

Certain improvements and modifications will occur to those skilled inthe art upon reading this specification. Clearly the basic conceptsdisclosed herein could just as easily be applied to an electrical valveactuator. It will be understood therefore that such improvements andmodifications were deleted herein for the sake of conciseness andreadability but are within the scope of the claims.

I claim:
 1. A valve actuator system comprising:pressure source meansincluding an accumulator charged to a predetermined pressure; pistonmeans connectable to a valve control member for providing linear motionin response to pressure applied from said pressure source means;positioner .[.means.]. for establishing control signals in response todifferences between input signals to said positioner .[.means.].indicative of desired valve positions and feedback signals to saidpositioner .[.means.]. indicative of actual valve positions; and meansfor moving said piston means at one of two speeds in response to themagnitude of the control signals from said positioner .[.means.].including a restrictor and by-pass assembly connected .[.between saidaccumulator and said piston means.]. .Iadd.to an exhaust line of a threeposition valve .Iaddend.to provide two different fluid flow volumesdepending on the magnitude of the control signals .Iadd.directly applied.Iaddend.from said positioner .[.means.]. .Iadd.to said three positionvalve.Iaddend..
 2. A valve actuator system as set forth in claim 1wherein said pressure source means includes:an air-actuated hydraulicpump; and said accumulator is connected to said air-actuated hydraulicpump to be hydraulically charged to said predetermined pressure by saidpump.
 3. A valve actuator system as set forth in claim 2 .[.includinga.]. .Iadd.wherein said .Iaddend.three-position valve .Iadd.is.Iaddend.connected between said .Iadd.piston means and said.Iaddend.accumulator and said moving means having a first positionresponsive to a first control signal for preventing flow from saidaccumulator to said piston means, a second position responsive to asecond control signal for allowing flow from said accumulator to oneside of said piston means to cause linear motion in a first direction,and a third position responsive to a third control signal for allowingflow from said accumulator to the opposite side of said piston means tocause linear motion in a second direction opposite said first position.4. A valve actuator system as set forth in claim 3 wherein saidpositioner .[.means.]. includes:input signal means for establishing asignal indicative of desired control valve position; feedback signalmeans for establishing a signal indicative of actual valve position; acontrol unit for comparing the signal from said input means with thesignal from said feedback means and applying the first control signal tosaid valve when said input signal is identical to said feedback signal,the second control signal to said valve when said input signal is lessthan said feedback signal, and the third control signal to said valvewhen said input signal is greater than said feedback signal, themagnitude of said second and third control signals being dependent uponthe difference between desired and actual control valve position.
 5. Avalve actuator system as set forth in claim 1 wherein said restrictorand by-pass assembly includes:a restrictor orifice mounted between saidaccumulator and said piston means; a by-pass line connected across saidrestrictor orifice; .[.and.]. a two-position valve mounted in saidby-pass line to .Iadd.be normally closed to .Iaddend.prevent flowtherethrough in a first position while allowing flow therethrough in asecond position.Iadd.; and a second two position valve also normallyclosed and smaller and quicker acting than said first two position valvewhich is larger to allow a large flow capacity through the by-pass lineand is connected to said second two position valve to control said firsttwo position valve in response to the control signals from saidpositioner being applied to said second two position valve.Iaddend...Iadd.6. A valve actuator system as set forth in claim 5 wherein thecontrol signals from said positioner are applied to said second twoposition valve through a selector switch connected between saidpositioner and said second two position valve. .Iaddend.